[SCSI] libfc: don't exch_done() on invalid sequence ptr

The lport_recv(), i.e., fc_lport_recv_req() may get called w/o the sequence ptr
being set in fr_seq(), particularly in the case of vn2vn mode, this may happen
if the passive fcp provider, e.g., tcm_fc, has not been registered yet.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/scsi/libfc/fc_lport.c

@@ -980,7 +980,8 @@ drop:
 	rcu_read_unlock();
 	FC_LPORT_DBG(lport, "dropping unexpected frame type %x\n", fh->fh_type);
 	fc_frame_free(fp);
-	lport->tt.exch_done(sp);
+	if (sp)
+		lport->tt.exch_done(sp);
 }
 
 /**